#include <stdio.h>

#define SIZE 500

int main()
{
    int n, m;
    scanf("%d%d", &m, &n);
    float a[SIZE] = {0.5, 0.5};
    int i, j;
    for (i = 2; i <= n; i++)
    {
        a[i] = a[i-1] + a[i-2];
    }
    float sum = 0;
    for (i = m; i <= n; i++)
    {
        sum += a[i];
    }
    printf("%.2f", sum);
    return 0;
}